The present invention generally relates to communication control units, and more particularly to a communication control unit which makes a communication in a communication network which has full duplex communication paths and wherein a source terminal sends a sending signal including a destination address and the communication control unit receives an answering signal from a destination terminal.
Presently, communication network systems employing optical fiber cables are being put into practical use because the optical fiber cables are less affected by noise caused by external electromagnetic waves when compared with coaxial cables. Such communication networks have full duplex communication paths. When making a communication in the full duplex transmission system using a carrier sense multiple access with collision detection (CSMA/CD) system communication protocol which is typified by an Ethernet (registered trademark), a star shaped local area network (LAN) and the like, the conventional collision detection system for the half duplex communication path cannot be applied for the detection of success or failure of the communication. In this case, another system must be used for the detection of success or failure of the communication.
A communication network system called ANARCHY is proposed in Neff et al., "A Local Network Design Using Fiber Optics", IEEE Comput. Soc. Int. Conf. 22nd, COMPCON Spring 1981. According to this proposed system, nodes constitute a communication network and the node detects a destination address within a sending signal which is transmitted from a source terminal to a transmission path. The node returns an acknowledge signal when the detected destination address coincides with an address of a terminal which is connected to the node. In other words, on the transmission side which sends the sending signal, it is discriminated that a collision occurred when the acknowledge signal is not received within a predetermined time from a time when the sending of the sending signal starts.
However, according to this proposed system, the degree of freedom with which the structure of the communication network may be designed is extremely limited because the node detects the destination address. For example, the address detection function of the node must be modified when the terminal is moved, changed or added. Accordingly, this proposed system suffers a drawback in that the modification of the terminal affects the entire communication network. In addition, the acknowledge signal is returned in response to the sending signal even after a communication path is fixed, thereby limiting the capability of the communication paths to the half duplex communication even though the communication paths are full duplex communication paths.
Generally, the communication control unit in the switching network generates a guard time for limiting the transmission of the signal for a predetermined time when the communication ends. The node which sets the communication path detects the end of the communication and releases the communication path when the signal transmission and reception in the communication path is stopped for over a predetermined time. After the communication ends, the terminal must generate the guard time and stop the transmission to the node. However, in the communication network disclosed in a U.S. Pat. No. 4,839,887, the sending signal (communication request signal) before the setting of the communication path is broadcast from the node, and not by a routing as in the case of the public communication network. The signals broadcast from the node is received by all communication control units, and only the communication control unit at the destination returns the answering signal. The communication control unit receives the sending signal which is not intended therefor every time a call out is made, and there is a problem in that the efficiency of the communication network becomes poor when the guard time is generated every time the sending signal is received.
In a discrete communication, the returning signal is returned in response to the sending signal (communication request signal). But in the case of broadcast communication, the destination address which is included in the sending signal is a broadcast address and does not designate one terminal. Hence, no answering signal is returned in response to the sending signal. For this reason, there is a problem in that the call out becomes unsuccessful when the sending signal and the returning signal are processed with the transmission procedure of the discrete communication.